1. Field of the Invention
The present invention relates to the field of plant biology and specifically to methods of modifying brassinosteroid response pathways and plant growth. An Arabidopsis gene called BZR1, variants, homologs and mutants thereof, appear to be involved in brassinosteroid responses. Expression of the BZR1 gene or the dominant mutant known as bzr1-D result in a variety of useful phenotypes such as increased cell expansion, plant size, yield, and cell size.
2. Description of the Related Art
The brassinosteroids are a unique class of biologically active natural products that possess plant steroidal hormone activity. Brassinosteroids play important roles in multiple plant developmental processes, and are primarily required for normal cell elongation and expansion. In light-grown plants, brassinosteroid deficiency results in dwarfism, while in dark-grown plants, brassinosteroid deficiency confers characteristics of light-grown plants.
Most multicellular organisms use steroids as signaling molecules for physiological and developmental regulation. Two different modes of steroid actions have been described in animal systems: the well-studied gene regulation response mediated by nuclear receptors, and the rapid non-genomic responses mediated by proposed membrane-bound receptors (Beato, et al., Cell 83:851, 1995; Mangelsdorf, et al., Cell 83:835, 1995; Wehling, Annu. Rev. Physiol. 59:365, 1997; and Schmidt, et al., Front Neuroendocrinol 21:57, 2000).
In both plants and animals, hormones typically act by binding to specific receptor proteins, thereby creating a ligand/receptor complex. The binding of the hormone to the receptor is believed to initiate an allosteric alteration of the receptor protein. In some instances, such as steroid receptors in animal systems, the ligand/receptor complex is capable of binding with high affinity to certain specific sites on DNA to modulate gene expression. The ligand/receptor complex may also act elsewhere in the cell. Recent evidence indicates that in addition to intracellular, genomic effects, steroids also exhibit non-genomic effects, ie., they affect the surface of cells and alter ion permeability, as well as release of neurohormones and neurotransmitters. Steroids such as estrogens and adrenal steroids and their naturally produced and synthetic analogs have shown membrane effects. In view of the foregoing, it appears that steroids may cause synergistic interactions between non-genomic and genomic responses resulting in multiple developmental and morphological alterations.
Brassinolide, one of the major brassinosteroids, was first isolated from the pollen of rape (Brassica napus) (Grove, et al., Nature, 281:216 1979), and was found to be a novel plant growth-promoting factor. To date, about 40 brassinosteroids have been found. Brassinosteroids are present at very low concentrations, and have been found to occur in all plant species examined (for review, see Mandava, et al., Ann. Rev. Plant Physiol. Plant Mol. Biol. 39:23, 1988).
Several mutations have been found to affect either light-dependent or hormone signaling pathways, resulting in plants with a dwarf phenotype. Brassinosteroid application can rescue some types of dwarf mutants in Arabidopsis since these mutants have been found to be defective in some aspect of brassinosteroid biosynthesis. Recent advances in our understanding of both brassinosteroid biosynthesis pathways and brassinosteroid response pathways have involved a combination of both mutational and biochemical analysis.
Several mutations in brassinosteroid biosynthetic pathways have been found. The Arabidopsis dwf7 mutant blocks biosynthesis of 5-dehydroepisterol from episterol, while the mutant dwf5 blocks 24-methylenecholesterol biosynthesis from 5-dehydroepisterol. The mutants dim, dwf1, and lkb block synthesis of campesterol from 24-methylenecholesterol. The synthesis of 5 α-campestanol from campesterol is blocked in the det2 mutant of Arabidopsis. The DET2 gene encodes a steroid 5 α-reductase. Overexpression of the DET2 protein increases brassinolide levels and results in larger, more robust plants. Later steps in the brassinosteroid biosynthetic pathway are blocked in the mutants dwf4 and cpd. Both the CPD and DWF4 genes have been found to encode cytochrome P450 proteins.
While many proteins have been found to participate in the biosynthesis of brassinosteroids, other proteins are involved in the coordination of plant responses to the presence of brassinolide or other brassinosteroids. Mutants in brassinosteroid perception are useful to study this aspect of brassinolide pathways, as several mutants have now been found which are blocked at some point in the brassinolide perception process. For example, bri1, cbb2, bin1, dwf2, lka, and cu-3 are thought to be blocked at some point between brassinolide synthesis and brassinosteroid-regulated responses.
Several of these mutants have been found to contain lesions in the Brassinosteroid-insensitive 1 (BRI1) gene. The BRI1 protein is a leucine-rich repeat receptor serine/threonine kinase, (Friedrichsen, et al., Plant Physiol. 123: 1247, 2000) containing a transmembrane region and an extracellular domain that functions as a plant brassinosteroid receptor at the plasma membrane surface (Wang, et al., Nature 410: 380, 2001; He et al., Science 288: 2360, 2000). BRI1 apparently becomes autophosphorylated upon exogenous application of brassinolide. Together, these findings suggest that the BRI1 protein is a brassinolide receptor kinase that transduces steroid signals across the plasma membrane (Wang, et al., Nature 410:380, 2001).
On a cellular level, brassinosteroids produce a variety of phenotypic changes in plants. For example, brassinosteroids accelerate seed germination and growth of seedlings, increase cell size and elongation, alter the arrangement of cortical microtubules and cellulose microfilaments, promote differentiation of xylem, promote leaf enlargement, increase plant dry weight, increase plant yield, induce H+ export and membrane hyperpolarization, promote tissue senescence in the dark, repress anthocyanin production in light-grown plants, and induce plant pathogen resistance responses to numerous bacterial and fungal species.
It is clear from the above discussion that brassinosteroids are an important group of plant modulatory molecules that play a role in many aspects of plant growth. Information leading to the further elucidation of brassinosteroid biosynthesis pathways and brassinosteroid response pathways is likely to be of agronomic importance. However, not all of the brassinosteroid pathway genes have been discovered. Thus what is needed in the art is a more complete description of brassinosteroid response pathway genes.